Vacuum dezincing of lead



Unite States VACUUM DEZINCING OF LEAD Leonard T. Cnrnow, Paul S. Randazzo, Arnold J. Skalak, and Thomas N. Bosiljevac, 'Jr., all of Omaha, Nebn, assignors to American Smelting and Refining Company, New York, N.Y., a corporation of New Jersey Filed Feb. 10, 1958, Ser. No. 714,364

4 Claims. (Cl. 75-78) This invention relates to a metallurgical procedure for refining lead under vacuum to remove zinc therefrom. More particularly, it relates to a batch process for vacuum dezincing of lead. V

The invention may be practiced with any lead which contains zinc and is particularly useful in treating the lead resulting from a well known desilverizing procedure in which silver contained in lead is removed therefrom as an insoluble zinc-silver compound by adding zinc to the lead. In accordance with this known procedure, the silver-containing lead is saturated at about 900 F. with Zinc and the thus saturated lead is cooled to its freezing point. After removing the insoluble zinc-silver compound which collects as a crust on the molten lead, and all of the insoluble compound which collects on the cooled lead, the residual lead still contains from 0.5 to 0.6% zinc which must be reduced in the subsequent refining of the lead to values below and usually well below 0.05% zinc. The principal advantage of the invention is that it affords a process for rapidly vol-atilizing zinc from lead. Another important advantage of the invention is that it provides a process in which the zinc contained in desilverized lead is efficiently and extremely rapidly reduced to values below 0.05% Zinc. These and other advantages will become apparent from the following more detailed description of the invent-ion.

In one aspect the invention comprehends volatilizing zinc from a molten pool of lead containing zinc by maintaining the pool under a vacuum while circulating the molten metal in the pool to and from the pool surface by impelling the metal on a path which comprises a radial outward flow of metal in all directions on the surface of the pool from a Zone on said surface toward the periphery of the pool and at a sufficient velocity to produce and maintain waves on the surface of the pool, and condensing the zinc volatilized from the pool.

The radial flow and the production and maintenance of waves on the surface of molten pool during the process are critically important in obtaining high rates of zinc removal from lead in practicing the invention. Preferably, the radial flow on the surface of the pool is obtained by releasing a confined stream of metal in an upward direction at the surface of the pool, or above or below the same; the velocity of the released stream being suflicient to produce and maintain waves on the surface of the pool. For best results, a confined stream is released in a vertical direction beneath and adjacent the surface of the pool.

The rate of zinc removal is further effected by the character and extent of the waves produced and maintained in the radially flowing metal on the surface of the pool. Thus, excellent results are obtained when the flow of the metal on the surface of the pool is such as to cause waves to occur on the major portion of the pools surface Better results are obtained when the waves occur over the entire surface of the pool. In either instance the results are improved when waves which are rough and irregular in shape are formed. A further a m 2,956,871 Patented Oct. 18, 1950 I 2 enhancement in results is obtained when the amplitude of at least some of the waves is such as to produce breaking waves, i.e. waves of such amplitude that the forward velocity of the wave crest is different from that of the body of the wave. In appearance and form such breaking waves are similar to white-capped ocean waves. In general it has been found that, when the waves possess amplitudes of at least about 3 inches in the vicinity of their source, breaking of at least some of the waves takes place.

Preferably a single vertically upwardly impelled confined stream, located in or adjacent and parallel to the vertical axis of the pool, is used. The upward velocity and the point of release of the confined stream are controlled to produce the desired wave action on the surface of the pool. Preferably, the confined stream is released at a fixed depth below the surface of the pool and at a sufficient velocity to produce waves having an amplitude of about 3 to 6 inches.

Where the radial surface flow of the invention is obtained by releasing an upwardly directed confined stream, a general circulation of molten metal centered on the confined stream is established. Thus, the molten metal from the confined stream flows radially outward in all directions on the surface of the pool from the zone on the pools surface corresponding to the point of release of the confined stream, and then flows downwardly and inwardly to the source of the confined stream, thereby continuously renewing the surface of the pool. With the preferred single vertically upwardly impelled and centrally located confined stream released from confinement beneath the surface of the pool, a turbulent mushroom-like mound of flowing molten metal is formed on the surface of the pool which is above the release point of the confined stream. From the mound the metal flows radially outwardly in substantially equal amounts in all directions toward the periphery of the pool, and then downwardly and inwardly to the source of the impelled stream, which source is preferably located deep in the pool.

Preferably, in conducting the process the atmosphere above the molten pool is maintained at an absolute pressure not in excess of about 1.0 mm., and most preferably from .01 to 0.1 mm. of mercury. Preferably also, the molten pool is maintained at temperatures from 900 to 1200 F. and most preferably at about 1070 to 1125" F. Under these conditions and especially under the most preferred conditions, undue volatilization of lead is avoided and the process is readily conducted in apparatus fabricated of ferrous metal, for example cast iron or steel.

The zinc volatilized from the pool may be condensed as a solid or a liquid in any desired manner, Preferably it is condensed on an extended surface adjacent the pool. For best results, especially in a batch process, the zinc is condensed as a solid on a water-cooled extended surface co-extensive with the surface of the pool and located about 8 to 14 inches above the pool when the latter is quiescent and under the operating vacuum.

The invention is further illustrated in the accompanying drawing and in the examples. It should be understood, however, that the drawing and examples are given for purposes of illustration and that the invention in its broader aspects is not limited thereto.

In the drawing the single figure is an elevational view in a cross section illustrating the preferred apparatus and procedure for conducting the invention as a batch process.

Referring now to this figure of the drawing, there is shown a round bottom steel kettle 1 in combination with removable vaporizing and condensing assembly 2. The kettle is mounted in furnace 3 which is provided with suitable fuel burner or burners (notshown). .The as sembly 2 is provided with steel bell member 4 in the form of a truncated, inverted hemisphere having an open bottom which is disposed a sufiicient distance below the flat condensing surface 5 of the bell to provide, as shown, a barometric seal for the bell when the latter is under the operating vacuum. The bell is provided with watercooled jacket 6 having water inlet and water outlet conduits 7 and 8. The bell is also provided with vacuum pipe 9 which leads from the interior of the bell and is detachably connected to conduit 10 which in turn leads to a conventional'vacuum pump (not shown).

In addition, the bell is provided with impeller 12 mounted on shaft 13, the lower portion of which is disposed in open-ended sleeve 14. Preferably the sleeve 14 extends from the top of the water jacket 6 downwardly at least for a distance which, in relation to the absolute pressure maintained in the bell during the process, is

sufiicient to provide a barometric seal at the lower end thereof. The shaft 13, as shown, is driven by motor 15 which is drivingly connected by means of gears 16, 17 and 18 to gear 19 mounted on shaft 13.

The impeller 12 is disposed in conduit 20 which is supported in position by straps 21 suitably attached to surface 5. The upper end of conduit 20 is disposed an appropriate distance from surface 5 to afford, during the operation of the process, discharge of molten metal from the conduit at, above or below, the surface of the molten pool and at the same time to provide an appropriate dis- H tance between the surface of the pool and the condensing surface 5. Preferably, as stated earlier, the space between condensing surface 5 and the surface of the pool when the latter is quiescent but under the operating vacuum in bell 4 is about 8 to 14 inches and the top of conduit 20 is disposed 12 to 18 inches beneath the surface of the pool. The assembly 2 is provided with supporting framework 22 which is carried on furnace 3 by means of legs 23 attached to the framework.

In operation, a charge of molten zinc-bearing lead is is introduced into kettle 1 from which assembly 2 has been removed, and the lead is skimmed to remove dross. The assembly 2 is then replaced on furnace 3 in the posi tion shown in the drawing, pipe 9 is reconnected to pipe 10, the Water connections to conduits 7 and 8 are made and water is circulated through jacket 6 at a sufficient rate to maintain surface 5 at a suitable temperature to condense solid zinc thereon. The lead is then brought to the operating temperature by appropriate firing of furnace 3. Thereafter the atmosphere above the metal in the bell is reduced to the operating pressure. As the bell is evacuated, metal is drawn into the bell from the kettle, thereby causing the level of metal in the former to rise and that in the latter to fall in response to the reduced pressure in the bell. However, loss of vacuum due to leakage of air into the bell is prevented inasmuch as the amount of lead charged to the kettle is such that the level 30 in the kettle is above the lip 31 of the bell when the bell has been evacuated to the operating pressure. Advantageously, especially with large size apparatus, the volume of lead charged to kettle 1 is substantially constant from batch to batch so that the level of metal in the bell during the dezincing of each batch will be substantially the same and at the desired distance from condensing surface 5 and the top of conduit 20 in each batch. Alternatively, varying amounts of lead may be charged to the kettle and means may be provided for raising or lowering assembly 2 on the furnace, for example by substituting adjustable studs for the fixed legs 23, to obtain the desired level of metal in the bell in each batch.

After thus readying this apparatus, the motor 15 which is adapted to rotate the impeller 12 in the direction indicated by the arrows, is started and operated at an appropriate speed to rotate the impeller 12 at a sufficient speed to impel the molten metal upwardly in conduit 20 to provide the flow features of the invention on the surface of the pool. Rotation of the impeller is then continued while maintaining the lead in the pool at the operating temperature and pressure, until the desired amount of zinc has been volatilized from the lead and condensed as a solid on surface 5. Thereafter the vacuum in the bell 4 is released, pipe 9 is disconnected from pipe 10, water circulation in jacket 6 is discontinued, and the assembly 2 is lifted from the kettle 1. The residual lead is re moved from the kettle and the zinc condensed on surface 5 is removed therefrom in any suitable manner, as by vibrating, chipping or melting. The above procedure may then be repeated with another batch charge of lead to be dezinced.

The drawing illustrates the metal flow on the surface and in the pool when conducting the process under the conditions which afford the best results in practicing the invention. As shown, the molten metal is impelled vertically upward as a confined stream in conduit 20 and at a speed such that when the stream is released from confinement it continues to move upward at a sufficient velocity to form the shown turbulent mound or so-called mushroom of flowing metal on the surface of the pool in the zone above the outlet from conduit 20. The metal then flows radially outwardly in all directions from the mound to the periphery of the bell and at a speed sumcient to produce rough waves, most of which are breaking waves, over the entire surface of the pool in the bell. The metal then flows downwardly and inwardly to the inlet to conduit 20.

In any given apparatus of the type illustrated in the drawing, the upward velocity of the molten metal in conduit .20 required to produce the radial and wave flow of metal on the surface of the pool in bell 4 in accordance with the invention is affected by the position of the discharge end of conduit 20 with respect to the level of the metal in the bell when the latter has been evacuated to the desired operating pressure. In any particular apparatus of this type, the necessary upward velocity for any given position of the discharge end of conduit 20 may be determined after the desired vacuum and metal level have been established in the hell 4, by adjusting the speed of motor 15 until the instant flow characteristics are established on the surface of the pool, as observed through a suitable fluid-tight window provided in the bell, for example a window through surface 5 which extends through jacket 6. However, the required upward velocity in conduit 20 may be conveniently predetermined by charging sufficient lead to kettle 1 so that the level of lead in bell 4 is substantially the same when the latter is not evacuated as the level will be when the bell has been evacuated to the operating pressure. A mirror attached to a rod and a light may then be lowered through pipe 9 after removing bend 32, and the speed of motor 15 may then be adjusted until the instant flow is produced on the surface of the pool in the bell, as observed in the mirror.

The invention is further illustrated in the following examples:

Example I metal was again heated to 1100 F., bell 4 was evacuated and motor 15 was started and operated at a fixed, predetermined, speed to volatilize zinc from the surface of the pool and condense the volatilized zinc on surface 5 during the dezincing treatment. Also during the treatment, the temperature of the lead was maintained at 1090 to 1100 F. and the pressure of the atmosphere in the bell above the pool was maintained at about 0.0l5

mm. of mercury. The level of the lead in the bell was 18 inches above the top of conduit 20 and 14 inches below surface 5 when the pool was quiescent and under the operating vacuum.

The flow of metal on the surface of the pool at the fixed, predetermined, operating speed of motor 15 was observed, as described above, by lowering a mirror attached to the end of a rod, and a light, through pipe 9. As observed in the mirror, the upwardly moving stream released from conduit 20 produced rough waves in the radially flowing metal. The waves moved across the pool, maintaining their rough and turbulent character, and lapped against the side of bell 4. It was also observed that, in the course of their radial movement to the periphery of the pool in the bell, most of the waves possessed breaking peaks.

The dezincing treatment was continued until the zinc content of the lead in the pool was reduced to 0.034%. It was found that this was accomplished in 3 hours. It was also found that the zinc which condensed on surface 5 contained 5% to lead and had condensed as a crystalline solid. This lead containing zinc was successfully used to desilverize additional portions of lead containing silver.

Example II 250 tons of desilverized lead containing 0.5 to 0.6% zinc were treated as described in Example I, using the same apparatus and conditions, including the same fixed, predetermined, operating speed for motor 15, as set forth in that example. It was found that the zinc content of the lead was reduced to 0.014% in 3.5 hours and that the zinc condensed on surface 5 contained 5% to 10% lead.

Example III 250 tons of desilverized lead containing 0.5 to 0.6% zinc were again treated as described in Example I, in the same apparatus using the same temperatures, pressures and level of metal in bell 4. In this example, however, the motor was reversed so that the molten metal was impelled downwardly from conduit by impeller 12. With this arrangement the metal flowed in a generally circular path as follows: downwardly in conduit 20, outwardly and upwardly along and parallel to the sides of bell 4, and inwardly across the surface of the pool in the bell into the top of conduit 20. It

was found that the surface of the pool remained substantially flat and that no waves could be produced thereon even when the motor was operated at full speed. It was also found that under these conditions, using full motor speed, at least 12 hours were required to reduce the Zinc content of the lead to 0.04%. It was found further that the zinc which condensed as a crystalline solid on surface 5 contained 5% to 10% lead.

What is claimed is:

1. A process for removing zinc from lead which comprises establishing a molten pool of lead containing zinc, maintaining the molten pool under vacuum while circulating the molten metal thereunder to and from the pool surface by impelling the metal on a path which comprises a radial outward flow of metal in all directions on the surface of the pool from a zone on said surface toward the periphery of the pool and at a sufficient velocity to produce and maintain waves on the surface of the pool, thereby volatilizing zinc from the pool, and condensing the thus volatilized zinc.

2. A process according to claim 1 in which said radial flow of metal on the surface of said pool is obtained by releasing a confined and upwardly impelled stream adjacent the surface of the pool, the upward velocity of said released stream being sutficient to produce and maintain rough and irregular shaped waves over the entire surface of the molten pool.

3. A process according to claim 2 in which the temperature of the pool is maintained in the range 900 to 1200 F., the absolute pressure above the pool is not in excess of about 1.0 mm. of mercury, a single stream of metal is impelled vertically upwardly adjacent the vertical axis of the pool and is released from confinement beneath the surface of the pool, and the upward velocity of the released stream is sufficient to produce waves having an amplitude of at least about 3 inches.

4. A process according to claim 3 in which said waves are breaking waves.

References Cited in the file of this patent UNITED STATES PATENTS 2,005,540 Fleming June 18, 1935 2,461,280 Isbell Feb. 8, 1949 2,786,755 Paddock et al. Mar. 26, 1957 

1. A PROCESS FOR REMOVING ZINC FROM LEAD WHICH COMPRISES ESTABLISHING A MOLTEN POOL OF LEAD CONTAINING ZINC, MAINTAINING THE MOLTEN POOL UNDER VACUUM WHILE CIRCULATING THE MOLTEN METAL THEREUNDER TO AND FROM THE POOL SURFACE BY IMPELLING THE METAL ON A PATH WHICH COMPRISES A RADICAL OUTWARD FLOW OF METAL IN ALL DIRECTIONS 